Revered as a symbol of hope, fidelity and purity, opal was dubbed the Queen of Gems by the ancient Romans because it encompassed the colors of all other gems. Opal is prized for its unique play of color , the ability to diffract light into flashes of rainbow color.

Opal occurs in different colors, ranging from semi-transparent to opaque. The most common is white opal. Crystal or water opal has a colorless body. The most valued variety, black opal, has a dark blue, gray, or black body color. Boulder opal combines precious opal with the ironstone in which it forms. Bright yellow, orange, or red fire opal is quite different from the other varieties of opal. Its day-glo tones, which are translucent to transparent, are beautiful with or without play of color.

Although opal is rarely enhanced by methods other than cutting and polishing, opals can be treated to bring out their play of color. One technique is to immerse white, gray or black opal in a sugar solution and then in strong sulfuric acid, which carbonizes with the sugar and leaves microscopic carbon specks that blacken the body color, making its flashes of color more visible. Opals can also be permeated with colorless oil, wax, resin, plastic, and hardeners to improve their appearance and durability. All of these forms of opal enhancement are stable. Fire opal is not commonly enhanced.